Marriage is not dead. According to an Ipsos Mori survey commissioned by the Civitas thinktank, 70% of people aged 20-35 in Britain want to marry. Rather than having to marry because of social pressure, or even for tax reasons, people are choosing to tie the knot in order to make a personal commitment. Living together is assumed to be the modern way, but eight in 10 cohabitees want to get hitched.

Marriage is dying, however, in the poorer parts of the country. Research based on the 2001 census shows the way in which unmarried parenthood is concentrated in Britain's most deprived areas. Married parenthood, by contrast, is the norm in the affluent areas, with high numbers of middle-class families.

Behind this disparity is not that the commitment of marriage is less valued among those on lower incomes, but that their circumstances impede its attainment. The significance of marriage today is what it represents: stable circumstances. The obstacles that poverty present to stability mean that, for many of the less well-off, marriage drops off the cards. In other words, marriage doesn't create stability - it signals it. What many of those on low incomes are missing are the prerequisites for this stability: employment and education.

Neither Labour nor the Conservatives, however, satisfactorily recognise the role of economics in family structure today. Both assume that people are not living in the married two-parent family through choice - "choices" signalling positive diversity to Labour, and declined values to the Conservatives. Non-marriage is therefore something to be embraced by Labour, and remedied through a married-couple allowance by the Tories. Strikingly out of touch with reality, these interpretations fail to acknowledge thwarted aspirations.

The comparative instability of cohabiting partnerships relates directly to the prevalence of single-parenting among low-income women. While 70% of children born within marriage will live with both parents until their 16th birthday, this is the case for only 36% of children born into a cohabiting union. The significance of single parenting, in turn, lies in the fact that child poverty is concentrated in single-parent households. In this respect, non-marriage and single parenting are directly connected to each other. As such, the government's disinterest in family structure under the nominally democratic "families come in all shapes and sizes" stance is highly problematic for its child poverty agenda.

To address the causes of single-parent poverty the government needs to support stability better, in both intact and separated families. Primarily, it should do this through employment, but also by recognising the impact of public policy on families.

As the Tories love to point out, low-income, two-parent families are treated less favourably than single-parent families. While the aim behind this is to support the poorest families, the effect is to risk creating more, by further undermining the stability of low-income, two-parent households.

The second cause of single-parent poverty is negligent non-resident fathers. The government's failure to enforce paternal responsibility - defying its gender equality agenda - again goes back to Labour's squeamishness about family structure. This extends much further than advocating the intact two-parent family. The two-parent structure ought to refer as much to the parenting structure as to the household. Treating fathers as optional extras is no longer liberating women; on the contrary, it is simply liberating men from their parental responsibilities.

· Anastasia de Waal is head of family and education at social policy thinktank Civitas, and is author of Second Thoughts on the Family, published by Civitas, £11.75 inc p&p. civitas.org.uk